Influenza viruses has already caused great disasters for humanity (Taubenberger and Morens 2007). Due to the lack of sufficient treatments and frequent mutations of the virus itself, the virus remains a threat to humans. In recent years, frequent and severe avian influenza epidemics as well as transmission of avian influenza to humans has constituted a great threat to health and economics of humans, so investigations directed to this kind of virus is of great value to protect human health. Avian influenza virus belongs to influenza virus A type, which are all me the members of the Orthomyxoviridae family. The virus genome consists of 8 negative sense single-stranded RNA. Through comparison and analysis of genes of influenza virus of avian origin and other influenza A viruses, sporadic mutations in the primary structure have been found, and these mutations result in differential pathogenicity of different influenza viruses. Now, it is established that the influenza virus genome encodes 11 proteins, wherein the replication of influenza virus genome RNA and mRNA transcription are dependent on a viral RNA polymerase which has become a potentially important drug target. Recent research suggested that the high pathogenicity of some influenza viruses is directly correlated with the polymerase mutations (Hulse-Post, Franks et al. 2007; Munster, de Wit et 20 al. 2007), further illustrating the necessity of designing drugs aiming at this complex. Investigation into this complex is of great significance to reveal the molecular mechanism underlying virus replication and to design drugs aiming at this complex. The RNA polymerase is a complex composed of PB1 (SEQ ID NO:2), PB2 and PA (SEQ ID NO:1) subunits, wherein PB1 (SEQ ID NO:2) is a subunit with catalytic activity, PB2 is responsible for acquiring cellular mRNA cap (CAP structure) through a snatching mode as primers of virus mRNA transcription, but PB1 (SEQ ID NO:2) acts as an endonuclease in this process. A temperature sensitive mutant ts53 suggests that PA (SEQ ID NO:1) takes part in the replication process of virus genome, but its specific function is still unclear (Sugiura, Ueda et al. 1975; Kawaguchi, Naito et al. 2005). The polymerase has three kinds of RNA activity which are needed in virus synthesis, i.e. mRNA, cRNA and vRNA synthesis, respectively. The mRNA synthesis starts from a capped oligonucleotide primer and ends 15-17 nucleotides prior to the vRNA terminal, followed by addition of a polyA tail. Polymerase can synthesize a cRNA intermediate of the full-length virus de novo and further synthesize full-length vRNA. Respective subunits of the polymerase can be expressed by an insect cell expression system, thus forming three different complexes, wherein one is a ternary complex containg the three subunits PB1 (SEQ ID NO:2)/PB2/PA (SEQ ID NO:1) of polymerase, and the other two are binary complexes: PB1 (SEQ ID NO:2)/PB2 and PB1 (SEQ ID NO:2)/PA (SEQ ID NO:1) binary complexes respectively, PB2/PA (SEQ ID NO:1) complex is not formed (Honda, Mizumoto et al. 2002). 25 amino acids of PB1 N-terminal are sufficient to interact with PA (SEQ ID NO:1) C-terminal, while the PB1 C-terminal is responsible for the interaction with PB2 N-terminal. A synthetic competitive small peptide of the PB1 N-terminal can significantly inhibit the activity of virus polymerase. RNA synthesis experiments using dinucleotide ApG as primers indicated that PB1 (SEQ ID NO:2)/PA (SEQ ID NO:1) complex can effectively initiate the replication of virus genome RNA, and PB1 (SEQ ID NO:2)/PB2 can synthesize virus mRNA in vitro (Honda, Mizumoto et al. 2002), but further studies in which the recombinant polymerase was expressed and purified using 293 cell revealed that all three subuits are necessary for replication and transcription (Deng, Sharps et al. 2006). The idea that PA (SEQ ID NO:1) principally participates in the replication process of virus RNA is derived from a finding that a tempreture sensitive mutant (L226P) can result in replication disorder of virus genome under non-permissible temperatures without affecting transcription activity (Kawaguchi, Naito et al.2005); whereas PB2 is involved in virus mRNA transcription. Further studies found that PA (SEQ ID NO:1) can extensively take part in processes such as transcription, replication and virus stability (Hara, Schmidt et al. 2006). The PB1 (SEQ ID NO:2)/PA (SEQ ID NO:1) complex can bind the 5′ terminal virus promoter, but PB1 (SEQ ID NO:2) by itself does not bind that promoter. Cross-linking experiments indicate that PA (SEQ ID NO:1) can bind vRNA and cRNA promoters (Fodor, Pritlove et al. 1994; Deng, Sharps et al. 2005; Hara, Schmidt et al. 2006) (Fodor 1994, Gonzalez 1999, Jung 2006, Hara 2006), but the specific binding sites are not clear. PA (SEQ ID NO:1) was found to have similar protease activity as chymotrypsins. Sanz-Ezquerro et al. (1996) found that about 250 amino acids at N-terminal are the active region of that protease—(Sanz-Ezquerro, Zurcher et al. 1996). But subsequently the studies by Hara et al. (2001) showed that the Serine at position 624 of C-terminal was the active site of PA (SEQ ID NO:1) protease, as mutation at that site resulted in loss of protease activity (Hara, Shiota et al. 2001). There is still controversy about the effect of PA (SEQ ID NO:1) protease activity on polymerase function. Hara et al. (2006) reported that purified recombinant PA (SEQ ID NO:1) protein can be degraded into two fragments with molecular weight of ˜25 kDa and ˜55 kDa through trypsin hydrolysis (Hara, Schmidt et al. 2006). It is known that understanding the three-dimensional structure of a protein is of great help to perform rational drug design, so identifying the three-dimensional structure of PA (SEQ ID NO:1) is of important value to perform drug design and function studies. In addition, previous studies have not reported the expression and purification of influenza virus protein PA (SEQ ID NO:1) in bacteria, so expression and purification of proteins in bacteria is of important benefit to further explore the function of PA (SEQ ID NO:1) and to perform drug screening, thus saving much time and greatly decreasing job cost and labour intensity.